Process for treating cast iron



1 cast iron having a white fracture.

Patented June 27, 1944 PROCESS FOR TREATING CAST IRON Franklin 13.

Rote, Ann Arbor, Mich., 'assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Application November 25, 1942,

Serial No. 466,939

3 Claims.

The present invention relates to cast iron, and more particularly to a method of treating or refining lead contaminated cast iron.

It has been found that cast irons which contain lead also frequently possess decreased mechanical properties and/or an undesired structure. In foundry practice accidental lead contamination of cast iron is frequently encountered, e. g., through the use of scrap or the use of cmcibles, covers, or other equipment which has previously been used for melting metals or alloys containing lead. In some instances considerable valuable alloyed scrap iron accumulated due to lead contamination in spite of many precautions to prevent it and there was no available method for reclaiming this material. It was found that a small amount of lead, e. g., about 0.004% or 0.005% up to about 0.02% or 0.03% lead, has a particularly deleterious effect upon the mechanical properties of cast iron sold under the trademark Ni-Resist. These cast irons are gray austenitic cast irons and usually contain about 12% to 17.5% nickel, about 5% to 7.5% copper and about 1.25% to 6% chromium, in addition to carbon, silicon, manganese, etc. An illustrative composition contains about 14% nickel, about 6% copper and about 2% chromium. Typical ranges for carbon, silicon and manganese are about 2.4% to 3.4% carbon, about 1% to 2.75% silicon and about 0.8% to 1.5% manganese. In a modification of these austenitic cast irons the copper is eliminated, or held below 0.5 and the nickel is raised to between about 18% and 22% with the chromium preferably falling within the range of about 1.75% to 3%. Up to a few hundredths of a per cent of lead caused the strength of these austenitic cast irons to drop, accompanied by a change in the form of the graphite. .A slight increase in the lead content resulted in the cast iron becoming white and unmachinable. The presence of very small quantities of lead in the aforementioned nickel-copper-chromium cast iron caused it to have an undesired fine dendritic structure. Thus, as little as 0.01% lead resulted in a marked tendency toward dendritic graphite formation. An addition of about 0.015% oflead resulted in a As little as 0.004% or 0.005% lead caused amarked reduction in strength. This is equivalent to about 1.3 or 1.6 ounces of lead per ton. The effect of lead upon the strength properties was noted for lead contents up to about 0.015% or 0.02%. With lead contents above about 0.02% the strength and other properties increased, probably as a result of the substitution of carbide for graphite. However, in a heat containing about 0.023% lead it was observed that although the strength increased, the impact resistance decreased. The presence of about 0.004% to 0.02% lead in the austenitic nickel-copper-chromium cast irons was found to lower the strength of uncontaminated cast irons which have a tensile strength of about 28,000 pounds per square inch down to as little as about 18,000 pounds per square inch and even less, the decrease usually being. greater as the lead content increases. Brinell hardnesses in some cases increases from about 195 to about 270.

- Some specifications for the austenitic nickel- I properties and/or structure of cast iron, particularly austenitic nickel-copper-chromium and nickel-chromium cast iron, contaminated with lead.

It is an object of the present invention to provide a process for improving the proper ies and/o1 structure of lead contaminated nickel-copperchromium cast iron,

It is another object of the present invention to provide a process for improving the properties and/or structure of lead contaminated gray cast irons, particularly austenitic nickel-cast irons.

It is a further object of the invention to provide a process for reclaiming lead contaminated cast irons, particularly nickel cast irons.

It is still another object of the invention to provide a process for overcoming the chill producing and carbide stabilizing effect of lead in lead contaminated cast irons, particularly austenitic nickel-copp'er-chromium and nickel-chromium castiron. I

Other objects and advantages of the invention will become apparent from the following description.

Generally speaking, the present inventionprovides a process for-treating lead contaminated cast irons, particularly austenitic nickel cast irons such as described hereinbefore, to improve the properties and/or structure of the cast iron, said process comprising treating molten lead contaminated cast iron with a mixture containingan alkali metal carbonate and an alkali metal nitrate,

preferably sodium carbonate and sodium nitrate. While other inertor harm-less ingredients might also be present. the essential ingredients of-the salt mixture are the carbonate and the nitrate. The salt mixture should contain about 30% to about 80% of the alkali metal nitrate, preferably sodium nitrate, and about 70% to about 20% of the alkali metal carbonate, preferably sodium carbonate. A salt mixture containing about 50% sodium nitrate and about 50% sodium carbonate has given satisfactory results. While it is by far preferred to use a mixture of the carbonate and nitrate, in some instances the use of the nitrate alone, as the essential ingredient, has some beneficial effect. However, the-use of the nitrate alone results in the formation of a small amount of watery slag which is hard to remove or hold in the container, e. g., the ladle, while pouring the molten cast iron.

In carrying the invention into practice, an addition of about 0.25% to about 4% of the salt; mixture, preferably a sodium salt mixture, is effective to counteract the deleterious effects of lead on the properties'of cast iron. While 'larger amounts of the salt mixture might be used, it is preferred to avoid an excess as it tends to cause chilling. About 0.5% to about 2%, e. g., about 1%, of a 50/50 mixture of sodium nitrate and anhydrous sodium carbonate has given satisfactory results. It is preferred to use anhydrous sodium carbonate, particularly in the treatment of the austenitic cast irons, e. g., the nickel-copper-chromium cast irons, because it has been found that the hydrated carbonate causes excessive cooling of the metal and the formation of larger quantities of slag. The dry salts cool the metal only slightly and form a smallamount of a fluid slag which can be hardened if desired by adding a, little silica, e. g., sand, to facilitate skimming. The salt treatment is preferably carried out in the ladle by making a ladle addition of the salt treating agent to the molten lead contaminated cast iron as it is tapped from the furnace. After suflicient time to allow the reaction or reactions to take place, the ladle is skimmed. The molten iron is then preferably treated with an inoculant, e. g., ferrosilicon, nickel-silicide, etc., to assure inoculation of the molten iron. The foregoing treatment with the oxidizing salt mixtureconverts lead contaminated, unmachinable, carbidic, white nickel-copper-chromium cast irons to a machinable, gray, higher strength austenitic cast iron whereas the customary known ladle inoculants which are also deoxidizers do not improve the structure or properties. Although the mechanical properties of the salt-treated lead contaminated cast iron may not always be raised bytreatment in accordance with the present invention to the' approximate levels of those of uncontaminated cast iron, a marked improvement in the properties and/or structure is obtained. Thus, increases of about 6,000 to 10,000 pounds per square inch in tensile strength are obtained in instances where lead contamination has caused at least as marked a decrease in the tensile strength.

In order that those skilled in the art may have a better lmderstanding of the invention the followingillustrative example is given.

EXAMPLE A charge of lead-contaminated nickel-copperchromlum cast iron sold under the trade-mark Ni-Resist" was melted down in a furnace. The

molten iron was tapped from thefurnace at about 2750 F. to about 2850" F. into a ladle. About 0.5% to 2% of the mixture containing equal parts of sodium nitrate and anhydrous sodium carbonate was added to the ladle and allowed to stand to permit the reaction to take place, e. g., for about one or two minutes. The ladle was then carefully skimmed and a customary deoxidizing ladle inoculant added, e. g., about 0.5% to 1% of silicon as ferro-silicon. Castings were then poured at about 2600 F. to about 2650 F.

Illustrative examples of the improved results obtained by treatment in accordance with the present invention as compared to untreated austenitic nickel-copper-chromium cast ons are set forth in Tables I and II.

Table I Per cent lead Per cent ladle addition Cast iron No B r A s It a ore er a treatment treatment mixture inoculant 0.02. 0.02..... 0.02. 0.006- None N one 0.015- 0.015.... 0.015- 0.013 0.02... 0.02 0.02..... 0.02.

Table II issi Tensile strength 5332 Fracture Dendritic. Fine grained gray. Fine grained gray. White plus some mettle. Fine grained gray.

Cast irons No. 1 and No. 2 had a total carbon content of about 2.8% and a silicon content of about 1.95%, in addition to nickel, copper and chromium in the usual amounts. Cast irons No. 3 to No. 5 were higher strength austenitic nickel-copper-chromium cast irons containing about 2.05% total carbon and about 1.6% silicon, and cast irons No. 6 and No. 7 were also higher strength austenitic nickel-copper-chromium cast irons, but contained about 2.2% total carbon and about 1.25% silicon. Cast irons No. 2 and No. 5 were treated in accordance with the present invention with a mixture of about equal parts of sodium nitrate and anhydrous sodium carbonate.

Cast iron No. '7 was treated with sodium nitrate alone. Cast irons No. 1, No. 4 and No. 6 were lead-contaminated cast irons which had the same composition as cast irons No. 2, No. 5 and No. 7, respectively. but were not treated with the salt agent in accordance with the present invention. Cast iron No. 3 was an uncontaminated cast iron having the same composition as cast irons No. 4 and No. 5, except for the absence of lead. It will be observed from east irons No. 1 and No. 2 that treatment in accordance with the present invention increased the tensile strength of the lead contaminated cast iron over 7,000 pounds per square inch'and converted the fracture from dendritic to a fine grained gray fracture. The lead contaminated cast iron No. 4 was converted from an unmachinable white cast iron having a Brinell hardness of about 269 to a fine grained gray cast iron having a tensile strength of about 34,200 pounds per square inch and a Brinell hardness of about 199 as compared to a tensile strength of 35,000 pounds per square inch and a Brinell hardness of about 196 for the uncontaminated cast iron illustrated by cast iron No. 3. Some. beneficial efiect is obtained by the use of sodium nitrate without carbonate as evidenced by comparing cast iron No. 7 with cast iron No. 6, but the use of the nitrate alone is not preferred.

It will be observed'that although treatment in accordance with the present invention markedly improves the properties of lead-contaminated cast iron and counteracts the bad eflects lead, it does not always greatly reduce the lead content. Thus, the treatment reduced the lead content of cast iron No. 2 from about 0.02% to about 0.006% whereas the treatment reduced the lead content of cast iron No. 4 from about 0.015% to about 0.013%. It is believed that by treatment in accordance with the present invention the lead content is oxidized, and in some cases apparently floated or slagged oil, but I do not desire to be limited to this theoretical explanation. Whatever the explanation, I have found that treatment in accordance with the process described hereinbetore improves the properties of lead-contaminated cast irons as compared to similar cast irons which have not been specially treated.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such variations and modifications are considered to be within the purview and scope of the invention and of the appended claims.

I claim:

1. A process for treating lead contaminated austenitic nickel cast iron which comprises treating a molten bath of said cast iron in a ladle with about 0.5% to about 2% of a salt mixture containing about sodium nitrate and about 50% anhydrous sodium carbonate, maintaining said salt mixture on said molten bath for sufficient time to allow reaction to take place, removing said salt mixture, introducing a siliconcontaining inoculant, and casting the treated molten cast iron. 1

2. A process for treating lead contaminated austenitic cast iron which comprises treating a molten bath of said cast iron in a ladle with about 0.5% to about 2% 01' a salt mixture containing about 30% to about 80% of sodium nitrate and about to about 20% of anhydrous sodium carbonate, maintaining said salt mixture on said molten bath for sufficient time to allow reaction to take place, removing said salt mixture, introducing a silicon-containing inoculant, and casting the treated molten cast iron.

3. A process for counteracting the deleterious efiect of lead in lead-contaminated cast iron which comprises treating a molten bath of said cast iron with about 0.25% to about 4% of a salt mixture containing about 30% to about of sodium nitrate and about 20% to about 70% of anhydrous sodium carbonate, maintaining said salt mixture on said molten bath, removing said salt mixture, introducing an inoculant in the treated molten bath and casting the treated molten cast iron.

FRANKIJN B. ROTE. 

